1. Field of the Invention
The present invention relates to a wide angle illumination optical system for an endoscope which makes possible utilization of illumination light without waste.
2. Description of the Prior Art
Since endoscopes of recent years have been equipped with wider angled observation system, it has become necessary that the illumination system therefor should also be of wider angle. The wider angle illumination system shown in FIG. 1, as an example, which has been developed from the above mentioned necessity, has been known. In this illumination system, a positive lens system 2 is disposed in front of a light guide 1 comprising an optical fiber bundle. In this arrangement, the illumination light coming from the light guide 1 is once converged and then dispersed by the positive lens 2 and, by utilizing this dispersed light, it has been made possible to obtain a wide angle illumination. However, illumination by this illumination system has a drawback in that a peripheral portion of the visual field lacks light. The reason for this is that in the case of such an illumination system the distribution curve of luminous intensity is as shown in FIG. 2 and, as the illumination angle becomes wider the light intensity becomes smaller and gradually reaches zero. Therefore, the effective illumination field in such an illumination system is narrower than the extent where the illumination light reaches. The portion which lacks light is in almost the same condition as the case in which no illumination light is present even if the illumination light has in fact reached there and, therefore, the light present at that portion is in effect wasted. As a method to remove such a drawback, an illumination method by which an exit end face of the light guide is projected (hereinafter referred to as "a projection type") is available. Accordingly to this method, an image of the exit end face 1a of the light guide 1 is formed on the object surface 4 by the lens 3 as shown in FIG. 3. In this example, the lens 3 is so selected that the size of the image of the light guide exit end face corresponds to that of the observation visual field and is so arranged to remove a parallax (as known, there exists a parallax since an image guide and a light guide are disposed side by side in an endoscope), and thus the observation visual field is completely illuminated and yet the illumination light does not reach areas other than the observation visual field and, therefore, no wasted light is present. Furthermore, since the exit end face of the light guide is projected, the distribution curve of luminous intensity is as shown in FIG. 4 and a certain luminous intensity is obtained even at the widest angle of view. Therefore, observation is possible over the entire area which the illumination covers and thus the illumination field and the visual field are coextensive. Further, the fact that the illumination light does not reach areas other than the observation field is advantageous not only in obviating waste of the illumination light but also in avoiding such an occurrence that the reflection light outside the visual field may form ghosts, etc. upon entering into the observation system causing a lack of clarity in the performance of observation. However, there are some defects in such a method as explained below. The exit end face of the fiber bundle as a whole does not uniformly illuminate but only the core portions of the respective fibers illuminate. Since the end face of the fiber is projected in its state onto the object surface, the object surface is illuminated with only the core portions being lit in patterns as illustrated in FIG. 5, that is, the object surface is illuminated as if it were covered by a net, which renders it extremely difficult to conduct a proper observation of the object. In practice, the net-pattern is blurred to some extent by virtue of the lens aberrations or other factors but such condition is not sufficient for carrying out the observation.